A conventional pulse width modulator generates a pulse width modulation signal by comparing an error signal with a linearly increasing or decreasing reference signal. This reference signal is generally a linear ramp signal, which has a constant slope and frequency. A voltage mode DC-to-DC power converter also uses a ramp signal. For example, as shown in FIG. 1, a conventional DC-to-DC PWM power converter 10 includes an error amplifier 12 to amplify the difference between an output voltage Vout and a reference voltage Vref to generate an error signal COMP, a ramp generator 14 to provide a ramp signal Vramp, a comparator 16 to compare the error signal COMP with the ramp signal Vramp to generate a pulse width modulation signal PWM, and a driver 18 to switch the power switches SW1 and SW2 according to the pulse width modulation signal PWM to convert an input voltage Vin into the output voltage Vout. The frequency of the ramp signal Vramp is the operating frequency of the PWM power converter 10, and the reciprocal of the slope of the ramp signal Vramp is related to the loop gain of the PWM power converter 10. In a typical voltage mode PWM power converter, the ramp signal Vramp has a constant slope; however, in some circumstances, e.g., at load transient, duly changing the slope of the ramp signal Vramp may increase the loop gain and thereby improve the transient response of the PWM power converter 10. This is the most important function of using a nonlinear ramp signal. The most common method is to use a multi-slope piecewise ramp signal so that the error signal COMP will touch different slopes of the ramp signal in steady state and transient state, respectively, in order to increase the loop gain to improve the transient response and speed up the transient response.
However, a problem exists for use of a multi-slope piecewise ramp signal, which involves the setting of a slope turning point between different ramps, i.e., the duty of the pulse width modulation signal PWM at a loop gain turning point. FIG. 2 is a diagram showing a two-slope ramp signal 20 having a slope turning point A. Assuming that the position C where an error signal COMP1 touches the first slope of the ramp signal 20 in steady state is far from the slope turning point A, it may be impossible for the error signal COMP1 to touch the second slope of the ramp signal 20 to increase the loop gain at load transient and consequently, the loop gain in transient state is just the same as that in steady state. In other words, the ramp signal 20 provides the same effect as a single-slope ramp signal. On the contrary, if an error signal COMP2 touches the second slope of the ramp signal 20 in steady state at the position B, then it will be difficult to adjust the stability of the whole loop because the second slope is set to get a higher loop gain and consequently improve transient response. In this case, it may cause the PWM power converter unable to operate stably. The duty of a PWM power converter is set by an external application circuit, a fixed slope turning point is unable to accomplish the desired effect.
U.S. Pat. No. 6,522,115 proposed to change the slope of the ramp signal by sensing the inductor current of a PWM power converter, which is based on the same principle as a current mode PWM power converter. However, sensing the inductor current requires an additional mechanism and thereby adds to complexity and cost of the circuit. Moreover, the resultant ramp signal has a concave curve waveform, which is inconvenient for compensation in voltage mode PWM power converters in different duty applications.